The introduction of WC in TiB2–TiC–CoNi cermets to form a core–ring structure contributes to strength and toughness

Abstract

In order to provide a more comprehensive study on the influence of WC on TiB₂–TiC–CoNi cermets, XRD, SEM, EPMA, TEM analysis, and hardness, three–point bending and indentation fracture toughness test were used to study the vacuum sintered TiB₂–25 TiC–20 CoNi–(0–12.5) WC cermets. The results indicated that TiC–(Ti, W, Co, Ni)C "core–ring" structure was formed in cermets added with WC. As the amount of WC added increased, the volume fraction of this (Ti, W, Co, Ni)C ring phase increased, resulting the drop of Rockwell hardness. However, the TiC–(Ti, W, Co, Ni)C exhibited fully coherent interfaces, which greatly enhanced the interfacial bonding strength between the ring and the core, thereby increasing the transgranular fracture, and improving the bending strength and toughness. As the added WC increased to 7.5 wt%, the bending strength and toughness gradually increased to peak values of 1697 MPa and 11.70 MPa m^1/2, respectively. When the added WC increased to 12.5 wt%, the bending strength and fracture toughness continued to decrease to 1443 MPa and 10.25 MPa m^1/2, respectively. The increased fraction of (Ti, W, Co, Ni)C ring phase, the formation of W–rich binder and decarburization compound W₃Co₃C, as well as the decrease in relative density might all contribute to this transformation.